The present invention relates generally to the synthesis of high-quality, colloidal nanocrystals, and more particularly to the use of metal oxides and metal salts as precursors to produce high-quality nanocrystals.
High-quality colloidal semiconductor nanocrystals are defined as nanometer sized, single crystalline fragments of the corresponding bulk crystals, which have a controlled size, distribution, and are soluble or dispersible in desired solvents and media. Semiconductor nanocrystals, especially cadmium telluride (CdTe), cadmium selenide (CdSe) and cadmium sulfide (CdS), are of great interest for fundamental research and technical applications due to their size and shape dependent properties and flexible processing chemistry. High-quality CdSe and CdTe nanocrystals with nearly monodisperse dots or elongated rods are actively being developed by industry as biological labeling reagents and for other applications such as LEDs.
Synthesis of high-quality semiconductor nanocrystals has a critical role in this very active field. The synthesis of CdSe nanocrystals using dimethyl cadmium (Cd (CH3)2) as the cadmium precursor has been well developed since first reported by Murray et al. [Journal of the American Chemical Society (1993), 115, 8706-8715]. Barbera-Guillem, et al. [U.S. Pat. No. 6,179,912] disclose a continuous flow process for the production of semiconductor nanocrystals using the method of Murray et al. One method for the synthesis of colloidal inorganic nanocrystals is the method developed for II-VI semiconductor nanocrystals as described in Peng et al. [Nature (2000), 404, 69-61] and Peng et al. [Journal of the American Chemical Society, (1998), 120, 5343-5344]. This synthetic method requires the use of metal precursors, such as dimethyl cadmium, which are extremely toxic, pyrophoric, expensive, and unstable at room temperature. At the typical injection temperatures (340-360° C.) required for nanocrystal synthesis using Cd(CH3)2 as the precursor, Cd(CH3)2 is explosive by releasing large amounts of gas. For these reasons, the Cd(CH3)2 related synthesis methods require very restrictive equipment and conditions and, thus, are not suitable for large-scale synthesis.
Monodispersity is another critical factor to be considered in synthesizing nanocrystals. Currently, CdSe nanocrystals are the only nanocrystals having a relatively monodisperse size distribution that can be directly synthesized by using dimethyl cadmium as the precursor. In the Journal of the American Chemical Society[(1998), 120, 5343-5344] Peng et al. reported that nanocrystal size and size distribution could be quantitatively determined by analyzing the growth kinetics of CdSe nanocrystals in a very hot non-aqueous solution. When the monomer concentration is controlled in the initial reaction solution, the size distribution of CdSe nanocrystals can reach close to monodispersity with a relatively low standard deviation (about 5%). This phenomenon is referred to as “focusing” of the size distribution. The size of the CdSe nanocrystals can be controlled by the amount of time allowed for growth. Recently, Peng et al. reported [Nature (2000), 404, 59-61] that the shape of CdSe nanocrystals can also be varied between dots (close to spherical shapes) and rods (elongated shapes). By comparison, the size and size distribution of CdTe and CdS nanocrystals cannot be controlled as well as the CdSe nanocrystals synthesized by the Cd(CH3)2 related method. There, thus, remains a need to develop a method for synthesizing high-quality semiconductor nanocrystals, whereby the size, size distribution, and shape of the nanocrystals can be well controlled during the growth stage.